1. Field of the Invention
An aspect of the present invention relates in general to an optical recording/reproducing apparatus, and more specifically, to an optical recording/reproducing apparatus capable of recording/reproducing information onto/from various types of optical recording media of different recording densities and different thicknesses.
2. Description of the Related Art
In an optical recording and/or reproducing apparatus for recording and/or reproducing information on and/or from an optical information storage medium, such as an optical disc, the recording density (or recording capacity) of the optical information storage medium is determined by the size of a light spot, using a light beam focused by an objective lens. Equation 1 below shows the relation between the size of a light spot S and a wavelength (λ) of light and a numerical aperture (NA) of the objective lens.S∝λ/NA  [Equation 1]
In general, in order to increase the area recording density of the optical disc, the size of a light spot formed on an optical disc needs to be reduced. As is evident from Equation 1, to reduce the light spot size, the wavelength should be short and the NA should be large. For instance, light with a short wavelength such as a blue violet laser and an objective lens with an NA of 0.6 or higher are required.
As well known already, a digital versatile disc (DVD) records and/or reproduces information using light with a wavelength of 650 nm (or 635 nm) and an objective lens with an NA of 0.6 (or 0.65 for a recording type DVD). Considering that a typical DVD is 120 mm in diameter and has a track pitch of 0.74 μm, its recording density on each side will be greater than 4.7 GB.
Nevertheless, the DVD is not sufficient for recording high definition moving images. This is because at least 23 GB of recording density for a single side is needed to record 135-minute-long moving images in high definition.
To meet the demand for high-density recording capacity, much research and studies are underway to develop and set standards of a high density optical disc, namely a next-generation DVD (hereinafter referred to as HD-DVD), or a Blu-ray, using light with a shorter wavelength than red light (i.e., 405-408 nm), such as blue light, and an objective lens with an NA of 0.6 or higher and having a narrow track.
Meanwhile, in order to secure a tolerance due to the tilt of the optical disc, the thickness of the optical disc should be reduced if the NA of the objective lens is increased for high-density recording. Therefore, in consideration of an allowable tolerance due to the tilt of the optical disc, the thickness of a conventional CD, i.e., 1.2 mm is reduced to 0.6 mm for a DVD and an HD-DVD. Also, the NA of an objective lens is increased from 0.45 for a conventional CD to 0.65 for a DVD and an HD-DVD. In case of the HD-DVD, a blue light source will probably be used in consideration of its recording capacity as a light source. Thus, a problem in the development of an optical disc based on the new standards is the compatibility of the new optical disc with the existing optical disc.
FIG. 1 is a schematic view of a related art optical recording/reproducing apparatus compatible with CD, DVD and HD-DVD using a single objective lens.
Referring to FIG. 1, the optical recording/reproducing apparatus includes first to third light sources 1a, 1b, 1c for use with various types of optical recording media of different recording densities and disk thicknesses, respectively; first to third optical path converters 2a, 2b, 2c for converting the path of a light emitted from each of the light sources 1a, 1b, 1c; a collimating lens 3 for collimating lights that passed through the optical path converters 2a, 2b, 2c; a reflective mirror 4 for reflecting lights that passed through the collimating lens 3 to the optical recording medium; a λ/4 wavelength plate 5 for changing polarization components of an incident light; an objective lens 6 for focusing lights that passed through the λ/4 wavelength plate 5 onto the optical recording media; and first and second photodetectors 7a, 7b for detecting information from a reflected light from the optical recording media. Here, the first light source 1a is for a CD (D1), the second light source 1b is for a DVD (D2), and the third light source 1c is for an HD-DVD (D3). Reference numeral 8 in FIG. 1 denotes a hologram element for diffracting and refracting light, and reference numeral 9 in FIG. 1 denotes a sensor lens for magnifying a spot of a light received to the first photodetector 7b. 
In the above-described optical recording/reproducing apparatus, the second and third light sources 1b, 1c emit lights for recording or reproducing information onto or from the DVD (D2) and the HD-DVD (D3) that require the same thickness and the same numerical aperture (NA). However, the first light source 1a must emit a light for recording or reproducing information onto or from the CD (D1) which has a different thickness and numerical aperture than the DVD (D2) and the HD-DVD (D3). Therefore, a different lens configuration having a different NA from the NA of the objective lens 6 is required, as shown in FIG. 1. As can be seen in the drawing, the first light source 1a is arranged closer to the collimating lens 3 than the second and third light sources 1b, 1c, so a light from the first light source 1a enters the objective lens 6 at a certain incidence angle.
Drawbacks of the optical system illustrated in FIG. 1 are that aberration of the objective lens 6 increases and the aberration increases even more due to the motion of the objective lens 6 under servo control. The increase in aberration produces an error on an information signal and thus an error signal.
Moreover, since the distances between the second and third light sources 1b, 1c and the optical recording media D1, D2, D3 are different from the distance between the first light source 1a and the optical recording media D1, D2, D3, two separate photodetectors 7a, 7b are required. Therefore, a total number of components increases, resulting in increased material cost.